1. Field of the Invention
The present invention relates to a power supply used for performing short-circuiting arc welding. The present invention also relates to an automatic welding machine or welder utilizing such a power supply. The power supply of the present invention is designed to perform xe2x80x9cnecking current controlxe2x80x9d and xe2x80x9celectrode polarity switching controlxe2x80x9d, as will be discussed later.
2. Description of the Related Art
As known in the art, arc welding is widely employed in the metalworking industries for joining two or more metallic parts. Taking the automotive industry for example, an assembly of metallic pieces are welded together for constructing e.g. a framework of a car seat. The welded framework may have a number of coalesced joints each having a welding length of no greater than 50 mm. Typically, such a framework is constructed from pipes, reinforcing plates, etc., that may be connected together by automatic CO2-arc welding. The thickness of materials (pipes, reinforcing plates, etc.) used for making the framework may vary from 0.7 mm to 2.0 mm. For reliable connection, the welds (that is, the coalesced joints) of the framework need to have a sufficient depth of fusion. It is also required that spatters generated during the arc welding operation will not stick to the obtained weldment. Generally, CO2-arc welding is a short-circuiting arc welding, and spatters tend to be generated when the short-circuiting occurs and breaks. In particular, when the short-circuiting breaks, a relatively great amount of spatters tend to be generated.
JP-B2-Heisei 4-4074 discloses a technique for preventing the occurrence of spatters. In accordance with the conventional method, a welding wire (consumable electrode) is positively charged (xe2x80x9celectrode positivexe2x80x9d mode) for performing short-circuiting arc welding, and the welding current Iw is controlled for the purpose of reducing spatters. FIG. 5 shows the waveform of the welding current Iw (Graph A), and illustrates how a molten globule 1a transfers from the welding wire 1 to the base material 2 (Phases Bxcx9cE).
As seen from FIG. 5, the short-circuiting between the welding wire 1 and the base material 2 occurs when the globule 1a formed on the welding wire 1 comes into contact with the molten pool 2a of the base material 2 (see Phase B) At this instant, the welding current Iw is decreased to reduce spatter generation.
Then, the welding current Iw is increased to exert a stronger electromagnetic pinching force on the molten bridging portion 2b (see Phase C). This promotes the globule transfer from the wire 1 to the base material 2.
Due to the increased pinching force, the molten bridging portion 2b is transformed into a constriction 2c (see Phase D). When the constriction 2c is detected, the welding current Iw is dropped off immediately before the regeneration of arc. This current-dropping process, called xe2x80x9cnecking current controlxe2x80x9d, can significantly reduce the amount of spatters to be generated at the reoccurrence of arc. For achieving the current-dropping, the DC output of the power supply is connected to a switching element, with a resistor connected to the switching element in parallel. When no constriction is detected, the switching element is held in the conduction state to short-circuit the accompanying resistor. Upon detection of the constriction, the switching element is turned off so that the DC power supply will be inputted to the resistor. Thus, the welding current Iw can be decreased rapidly.
When the constriction 2c is formed, the resistance of the molten bridging portion 2b increases. This resistance rise is reflected in the rate of the welding voltage (dv/dt) and the rate of the welding current (dR/dt). Thus, the above-mentioned detection of the constriction 2c can be performed by monitoring the rate dv/dt or dR/dt.
After Phase D in FIG. 5, the regeneration of arc 3 occurs (see Phase E). To maintain the arc 3, the welding current Iw is increased.
During the short-circuiting period, the welding current Iw needs to be controlled precisely. Thus, the external characteristic of the power supply should be constant current characteristic. During the arc-generating period, on the other hand, the power supply is caused to have constant potential characteristic so that the arc has a proper length.
While the above-described prior art technique contributes to the reduction of spatters, it still may suffer a drawback illustrated in FIGS. 6A and 6B. Specifically, in these figures, two metal plates are about to be welded together. Each of the plates has a relatively small thickness (below 1 mm, for example). Further, some unavoidable gap is present between the upper and lower plates. When the prior art welding is performed with the thin plates, the lower plate may be formed with burn through holes due to an excessive amount of heat input.
Reference is now made to FIG. 7 showing the waveform of a conventional welding current Iw (see JP-A-Heisei 11-226730, for example). The welding current Iw for the Tep period is supplied in the state of the xe2x80x9celectrode positivexe2x80x9d, while the current Iw for the Ten period is supplied in the state of the xe2x80x9celectrode negative.xe2x80x9d In accordance with this prior art, it is possible to achieve a shallow depth of fusion by appropriately setting the cycle T (=Tep+Ten) and polarity rate Tep/(Tep+Ten) of the alternating welding current Iw. Accordingly, the xe2x80x9cburn through holexe2x80x9d problem illustrated in FIGS. 6A-6B can be avoided. However, this alternative prior art method tends to produce a great amount of spatters at the time of the occurrence and breakage of the short-circuiting between the welding wire and the base material.
The present invention has been proposed under the circumstances described above. It is, therefore, an object of the present invention to provide a power supply for performing spatterless short-circuiting arc welding without causing burn through holes to be made even in a relatively thin base material.
According to a first aspect of the present invention, there is provided a power supply for short-circuiting arc welding. The power supply comprises: a primary power circuit for output of direct voltage; a primary controller for controlling the output from the primary power circuit and feeding of a welding wire; a polarity switching circuit including both an electrode positive switching element and an electrode negative switching element for selectively supplying an arc load with either one of electrode positive and electrode negative voltages based on the direct voltage from the primary power circuit; a first unit including a first switching element and a first resistor connected to the first switching element in series, the first unit being connected in parallel to the electrode positive switching element; a second unit including a second switching element and a second resistor connected to the second switching element in series, the second unit being connected in parallel to the electrode negative switching element; a necking determination circuit for outputting a necking determination signal by determining, based on at least either one of a welding voltage variation and a welding current variation, whether a constriction occurs in a molten bridging portion extending between the welding wire and a base material; a driving circuit for bringing the electrode positive switching element into a conduction state only when an electrode positive polarity setting signal is inputted from outside the power supply and no necking determination signal is outputted; a driving circuit for bringing the electrode negative switching element into a conduction state only when an electrode negative polarity setting signal is inputted from outside the power supply and no necking determination signal is outputted; a driving circuit for bringing said first switching element into a conduction state for rapidly decreasing the welding current only when the electrode positive polarity setting signal is supplied and the necking determination signal is outputted; and a driving circuit for bringing said second switching element into a conduction state for rapidly decreasing the welding current only when the electrode negative polarity setting signal is supplied and the necking determination signal is outputted.
According to a second aspect of the present invention, there is provided an automatic welding machine incorporating the above-described power supply. The welding machine further comprises a welding robot; and a robot controller. The robot controller controls the movement of the welding robot and also sends welding condition setting signals to the power supply. The welding condition setting signals may include a polarity setting signal suitable for a welding area of the base material.
Other features and advantages of the present invention will become apparent from the detailed description given below with reference to the accompanying drawings.